SEMINAR: Spatial Variability in Transitional Response of Vegetation Indices to Surface Temperature

Vegetation activities sensitively responsd to topological conditions and a changing climate. To better understand vegetation dynamics under varying environmental conditions, we analyse observations of vegetation indices, Land Surface Temperature (LST), air temperature, and precipitation of a deciduous broadleaf forest in Korea over an eleven year period from year 2000 to 2010. While a positive relationship between EVI (Enhanced Vegetation Index) and LST is established for all seasons, independent of elevation, the EVI-LST relationship exhibits a threshold above which the rate of EVI change over LST changes significantly. This transitional point is associated with the condition that the air temperature triggers the start of the growing season, while soil moisture is more critical during the growing season. The EVI-LST relationship above the threshold exhibits a hysteresis loop, i.e., rising limb differs from falling limb. We identify two types of hysteresis patterns, i.e., a single loop and a twisted loop. The variation in loop width between the ascending and descending paths contains a signature in annual precipitation from the concurrent year. In our analysis, annual precipitation above 1,600 mm is associated with an LST difference of more than 5 oC in the loop width.

The analysis was extended to Bosnia, West Virginia (USA), and Tasmania (Australia) with similar climate regimes i.e. temperate but varying in rainfall patterns and vegetation composition. Inter-annual EVI-LST plots in Kor, Bos, and WV exhibit a similarity in the threshold, but the threshold is absent in the Australian sites. This trait is attributed to extreme low winter temperature in Kor, Bos, and WV, but all sites essentially exhibit a similar response rate, which suggests that vegetation in similar climate regimes displays a convergence in response to temperature, independent of species composition. The annual EVI-LST relationship exhibits hysteresis loops at all study sites. The shapes of the loops well depict annual rainfall variability in Kor but not in Bos and WV. The loop trajectories are very likely to display a photosynthetic response to moisture availability. This response pattern in the EVI and LST relationship further highlights the sensitivity of EVI to assess biophysical conditions of vegetation.